A modern probe card used to test wafers of semiconductor chips generally consists of a Printed Circuit Board (PCB) (also referred to as a printed wiring board or probe card wiring board), a probe contactor substrate having probes for contacting the wafer (sometimes referred as a probe head), and an interposer connecting the PCB to the probe contactor substrate.
Probes (also referred to as probe contactors) are generally compliant mechanisms including at least one spring which have some limited range of compliance in a vertical direction (the “z” direction). When in use, a wafer under test is urged upward to contact the tips of the probes. In practice, there is some manufacturing process-related z error (non-planarity of the probe tips) caused by film stresses, etch control, assembly control, etc. as well as systemic z errors caused by a warping or curving in the surface of the probe contactor substrate. If the probe contactor substrate is curved or warped, so will be the imaginary plane defined by the tips (assuming that the probes are of uniform height). Thus some probe tips will contact the wafer first (called the first touch z height) and some probe tips will contact the wafer last (last touch z height). Because probes generally have a limited range of compliance (as small as 50 μm or less for many microfabricated technologies), it is desirable to minimize both the process-related and systemic errors in tip z height. Some errors are most directly related to the fabrication of the probes on the probe contactor substrate rather than the probe card design. And some errors are directly related to the probe card and the way the PCB is mounted with the probe contactor substrate or substrates.
Referring to FIG. 7A, a common problem with mounting the probe head to the probe card, is that the probe contactor substrate 700 to which the probe contactors 705 are attached is generally non-uniform and has thickness variations across its surface. (While not shown in FIG. 7A, the probe contactor substrate may also have variations in the in the thickness in the direction orthogonal to the plane of the page.) The probe contactor substrate 700 is generally mounted to the probe card 710 in such a manner that the planarity is set by the location of discrete points along the back surface (the surface opposite of the probe contactors) of the probe contactor substrate 700 as shown in FIG. 7B; thus, due to the non-uniform thickness of the probe contactor substrate 700, the plane 715 of the probe contactor tips may not be co-planar with the plane 720 of the reference plane. The probe contactor may also be mounted to the probe card in such a manner that the planarity is set by the location of discrete points along the front surface of the probe contactor substrate. In this case, tolerances in the probe card as well as other non-planarities can cause the plane of the probe contactor tips to be non-co-planar to the reference plane. It is a further problem that because of manufacturing tolerances or errors, the overall heights of the probe contactors can vary linearly across the substrate such that when referenced to the reference plane (i.e., 720), the probe tips lie in a plane that is tilted.
In older probe card applications, a prober has a surface which has been planarized to that of the chuck that carries the wafer under test. The probe card PCB is generally mounted to this planarized surface of the prober. Thus, all such probe card assemblies require well controlled parallelism between the plane of the probe tips (the best-fit plane that minimizes the overall root-mean-square z error between the probe tips and the plane) and the plane of the PCB (the PCB can be thought of as the “reference plane.” If the probe tips are co-planar with the PCB, then they are also co-planar with the chuck, and thus with the wafer under test). Such a design will lead to a more uniform contact of the probes to the wafer under test (less of a distance between first touch z distance and last touch z distance). However, this older type of mounting is unable to support the high probing forces or probing tolerances required in modern high precision and high pincount probing applications. In newer probe cards, the probe tips are referenced to mounting points on the probe card which are typically kinematic mounts of some type (used here to describe a mount that provides accurate and repeatable mechanical docking of the probe card into the test equipment and provides constraint in at least the three degrees of freedom necessary to achieve parallelism to the plane of the wafer chuck). In either embodiment, it is necessary to align the tips of the probe contactors such that they are parallel to a reference plane which is itself parallel to the plane of the wafer chuck.
There are two common ways that a probe contactor substrate may be mounted to the probe card (which includes the PCB, an associated stiffener ring and/or other mechanical elements) in a planar manner: Fixed Probe Card Assemblies (FPCAs) and Adjustable Probe Card Assemblies (APCAs). FPCAs provide for design simplicity (no moving or adjustable parts) and relatively low cost. However, the machining tolerances required for parallelism, particularly in the case of large area probe cards, can be difficult to achieve. Hence, in practice, shims are often used to provide some degree of planarity tuning during assembly. Shimming, though a practical alternative, is difficult to perform accurately in a manufacturing environment to the tolerances required (on the order of microns). Also, shimming is not a practical means of adjusting planarity in the field to suit the planarity errors or offsets of a particular test cell or group of test cells.
FPCAs include U.S. Pat. No. 3,806,801 entitled “Probe Contactor Having Buckling Beam Probes.” Buckling Beam assemblies have a vertical buckling beam probe head, a PCB, and an interposer situated between the probe head and the PCB. In this case the interposer comprises an array of solder balls which electrically connect the substrate to the PCB (terminal to terminal) but other examples are well known in the art where the interposer connects the substrate to the PCB by means of spring-pins or other compliant means. In another version of the Buckling Beam probe card, there is no interposer and the buckling beams connect directly to the terminals of the PCB.
APCAs allow for some adjustment. U.S. Pat. No. 5,804,983 entitled “Probe Apparatus with Tilt Correction Mechanisms” and U.S. Pat. No. 5,642,056 entitled “Probe Apparatus for Correcting the Probe Card Posture Before Testing” describe an APCA. As shown in FIG. 8, the apparatus contains a plate head 17 and an insert ring 18 (also known as a probe card mount). An adjuster is located between them at part 53. The adjuster allows an adjustment of the card body 24, and consequently the probe contactors 23. In practice, the adjuster 53 is manipulated to make the insert ring 18 and the probe chuck 15 parallel. This method is based on the often faulty assumption that the probe card's reference plane is parallel to the contactor tip plane, such that making the insert ring 18 parallel will automatically render the probe tips parallel to the probe chuck 15.
In practice, the probe testing apparatus is adjusted periodically (for example, once a year) because it requires that the apparatus be taken off line for a long period. The adjustment is generally not made with regards to a certain card, again based on the assumption that the probe card's reference plane is parallel to the contactor tips. Thus, when the probe testing apparatus is fitted with a new probe card, often the adjustments are left intact. As described above, this may result in a planarity error that is attributable to the manufacturing variance of the probe cards themselves.
Other methods to fine tune planarization have been used. For example, U.S. Pat. No. 5,974,662 entitled “Method of Planarizing Tips of Probe Elements of a Probe Card Assembly,” shown in FIG. 9, describes such an APCA and discloses a method of making a probe card with an adjustment of the probe tips relative to the probe card's reference plane. The probe card incorporates a space transformer substrate which is mounted to the probe card in such a way that the orientation can be adjusted. A vertical spring interposer is used to electrically interconnect the probe contactor substrate to the PCB and differential screws bearing on the substrate provide the adjustability. This design is particularly expensive, difficult to assemble, and complex. In addition, the large number of mechanical components required to achieve adjustability make the design inherently thermally and mechanically unstable.
Another fine tuning method is shown in FIG. 10, which is taken from U.S. Pat. No. 6,609,751 entitled “Planarizer for a Semiconductor Contactor.” The probe card substrate 210 has a stud 238 attached to it. By turning the actuating nut 242, the stud 238 can pull or push on the substrate 210. Additionally, screws 224 can exert force on the substrate 210. The mechanism can cause the substrate to bow in order to planarize the probe contactors 211. This design, however has many disadvantages. First, the design is complicated and requires that the substrate float on ball bearing and springs. Second, bowing the substrate 210 places the substrate 210 under enormous stress, and given that the substrate 210 is made of a brittle and relatively fragile ceramic, this stress can break the substrate 210—rendering the probe card completely inoperable. Third, bowing the substrate 210 also stresses the interposer 230 and can cause some of the contactors 229 from the interposer 230 to (1) misalign with their intended pads, or (2) if the bowing is severe enough, completely disengage from their intended pads. In either case, the probe card would not be in complete and accurate electrical connection with the probe head and, thus, the diagnostic equipment and would render the probe card (or at least a portion of the probe card) inoperable.
What is needed, therefore, is an improved probe card that allows for fine tuned planarization. Moreover, the probe card must be less complicated and expensive, and cannot impart undesirable stresses on the substrate or the interposer.